1. Field of Invention
The present invention relates to an adhesive composition useful for making a repositionable sheet. The adhesive composition is comprised of acrylate or acrylic polymers having two different particle size distributions. The composition of the invention enables the properties of the adhesive to be varied to meet the particular adhesive specifications for a specific application.
2. Description of Related Art
Polymerizing acrylates and methacrylates to form adhesives is well known in the art. The prior art has long sought adhesive systems that enable the level or amount of tack to be easily varied and controlled. With the constant demand for repositionable labels and sheets, the art is in search of an adhesive that can be used for various types of repositionable labels or sheets. The present invention addresses these needs by providing an adhesive which controls the tackiness, thus making it possible to produce adhesives having the appropriate tack level required in any specific application. In addition, the high costs of reformulation and possible process changes for different products can be avoided with the adhesives of the present invention.
In accordance with the present invention, mixing different-sized adhesive particles produces significant, beneficial results without the difficulty of having to change other parameters, e.g. coat weight or delamination value, for each different batch of adhesive made. The mixture combines characteristics: the adhesive strength of the larger particles with the ease of removal of adhesive from a surface to which it is adhered inherent in the smaller particles.
Adhesive compositions that can be used to produce a repositionable sheet are known in the art. However, such adhesive compositions in the prior art have been composed of polymer components as well as other ingredients such as cross-linkers or fillers. The presence of components other than the polymers can change the properties of the entire batch of adhesive. Adhesive strength and performance may be affected, for example. Therefore, when adhesive compositions contain these other ingredients, the adhesive generally has to be reformulated for each different product line, adding greatly to the manufacturing cost.
U.S. Pat. No. 4,645,783 to Kinoshita discloses a repeatedly usable and releasable sheet coated with an adhesive composition comprising microspherical copolymer particles (A) with an average diameter of 10-100 microns prepared by aqueous suspension polymerization of alkyl acrylate esters, .alpha.-mono-olefin carboxylic acids, and vinyl monomers; and polymer microparticles (B) with an average diameter of 0.1-4.0 microns prepared by polymerization of vinyl monomers in an aqueous medium. The adhesive composition of Kinoshita also contains other ingredients, such as surfactants and a protective protein colloid. In order for Kinoshita to achieve the requisite small size of the (B) particles, a large amount of energy had to be expended.